Having seen a video[1] of a drone armed with a paintball pistol (and its precision), I would say that it will be much sooner than expected. Despite the fact that I'll probably be put on a watch-list for saying this, it would be the perfect crime. There would be no weapon left at the scene (provided the drone returns to its base), as well as no DNA samples. Anyone who could rig and pilot such a drone would probably be smart enough to leave minimal evidence related to motives, purchases, etc.
